Thermoplastic olefins (TPOs), impact copolymers (ICPs) and thermoplastic vulcanizates (TPVs), collectively referred to herein as “heterogeneous polymer blends”, comprise an isotactic polypropylene thermoplastic phase and a high molecular weight or crosslinked elastomeric phase. These heterogeneous polymer blends also commonly include non-polymeric components, such as fillers and other compounding ingredients. The heterogeneous polymer blends have multiphase morphology where a thermoplastic such as isotactic polypropylene (often referred as the hard phase) forms a continuous matrix phase and the elastomeric component (often referred as the soft phase), generally derived from an ethylene containing copolymer, is the dispersed component. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO, while the ethylene copolymer imparts flexibility and impact resistance.
TPOs and ICPs are typically made during the polymerization process by differential polymerization of the polymer components, although some can also be made by mechanical blending. TPVs are also blends of thermoplastic and elastomer, like TPOs, except that the dispersed elastomeric component is crosslinked or vulcanized in a reactive extruder during compounding. Cross-linking of the elastomeric phase generally allows dispersion of higher amounts of rubber in the polymer matrix, stabilizes the obtained morphology by preventing coalescence of rubber particles, and enhances mechanical properties of the blend.
Traditionally, the elastomeric component in heterogeneous polymer blends has been provided by highly amorphous, very low density ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) having a high molecular weight. Recently, other ethylene-alpha olefin copolymers have been used, especially very low density ethylene-butene, ethylene-hexene and ethylene-octene copolymers which generally have a lower molecular weight. The density of these latter polymers is generally less than 0.900 g/cm3, indicative of some residual crystallinity in the polymer.
The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. Other applications include automotive interior components such as door skins, air bag covers, side pillars and the like. These parts are generally made using an injection molding processes. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate>35 dg/min.). These high melt flow rate (MFR) resins are low in molecular weight and consequently difficult to toughen, resulting in products that have low impact strength. It would be desirable to have a polymer blend with greater elongation to break and more toughness, improved processability, and/or a combination thereof.
In addition, in-reactor blends have been sought as an alternative to physical blending since in-reactor blends offer the possibility of improved mechanical properties through more intimate mixing between the hard and soft phases, through the generation of hard/soft cross products, as well as lower production costs. Use of compatibilizer is another way to improve interfacial tension between hard and soft phases in the heterogeneous blend, thereby improving the mechanical properties.
Thus, known compositions have low molecular weight and low toughness or may have a higher molecular weight (low melt flow rate) in order to meet the required mechanical strength for the targeted applications but lack processibility. Blends with both good mechanical properties and high melt flow rate as well as processes to produce them would be useful in the art, especially for applications such as injection molding.